A fuel cell, for example a polymer electrolyte fuel cell, is an apparatus that allows a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air to electrochemically react with each other at a gas diffusion layer that has a catalyst layer such as platinum, such that electric power and heat are produced at the same time.
FIG. 7 is a schematic diagram showing the basic structure of a conventional polymer electrolyte fuel cell. A single cell (also referred to as a cell) 100 of the polymer electrolyte fuel cell includes a membrane electrode assembly 110 (hereinafter referred to as the MEA: Membrane-Electrode-Assembly) and paired plate-like conductive separator 120 disposed on opposite faces of the MEA 110, respectively.
The MEA 110 includes a polymer electrolyte membrane (a resin ion exchange membrane) 111 that selectively transports hydrogen ions, and paired electrode layers 112 formed at the opposite faces of the polymer electrolyte membrane 111. The paired electrode layers 112 are formed at the opposite faces of the polymer electrolyte membrane 111, and each includes a catalyst layer 113 which is mainly comprised of carbon powder bearing a platinum metal catalyst, and a gas diffusion layer 119 (also referred to as a GDL) that is formed on the catalyst layer 113 and that has combination of features of current-collecting function, gas permeability, and water repellency. The gas diffusion layer 119 is structured with a base member 115 made of carbon fibers, and a coating layer (a water-repellent carbon layer) 116 structured with carbon and a water-repellent member.
The paired separators 120 are provided with, at their main surfaces abutting on the gas diffusion layers 119, respectively, fuel gas flow passage grooves 121 for allowing the fuel gas to flow through, and oxidant gas flow passage grooves 122 for allowing the oxidant gas to flow through. Further, the paired separators 120 are provided with coolant flow passage grooves 123 through which coolant or the like passes. Supply of the fuel gas and the oxidant gas to the paired electrode layers 112 through the gas flow passage grooves 121 and 122, respectively, causes an electrochemical reaction, to produce electric power and heat.
As shown in FIG. 7, the cell 100 structured as described above is generally used by being stacked by one piece or more, so that the cells 100 adjacent to each other are electrically connected in series. It is noted that, here, the cells 100 stacked together are fastened under pressure at a prescribed fastening pressure by fastening members 130 such as bolts, so as to prevent leakage of the fuel gas and the oxidant gas and to reduce the contact resistance. Accordingly, each of the MEAs 110 and each of the separators 120 are brought into plane-to-plane contact at a prescribed pressure. Further, in order to prevent the gases required for the electrochemical reaction from leaking externally, a sealing member (gasket) 117 is disposed between the separators 120 and 120 so as to cover the side surface of the catalyst layer 113 and that of the gas diffusion layer 114.
In connection with the polymer electrolyte fuel cell structured as described above, the gas diffusion layers 114 of a variety of structures are known. For example, Patent Document 1 discloses a gas diffusion layer similar to the conventional gas diffusion layer 114 described above, in which a coating layer (a water-repellent, carbon layer, a C layer) made up of a carbon material and a water-repellent member is provided on the surface of a base member (for example, paper, woven fabric, or nonwoven fabric) made of carbon fibers. Further, Patent Documents 2 and 3 each disclose a gas diffusion layer which does not employ carbon fibers as the base member. The gas diffusion layer of Patent Document 2 is structured by having a mesh sheet processed with a water-repellent material, and having the void portions of the mesh sheet filled with a paste made up of carbon and a water-repellent member. The gas diffusion layer of Patent Document 3 is structured with a mixture of graphite, carbon black, uncalcined PTFE (polytetrafluoroethylene), and calcined PTFE.